Ask Mr. Smarty Plants

First, it is great that you are planning to use a Texas mountain laurel (Sophora secundiflora) in your new garden. Even though they grow quite slowly, they can get tall. So watch how close you are planting it in front of your window if they are low to the ground.  Here are some details from our website. Mescal bean or Texas mountain laurel is an evergreen; usually multi-trunked shrub or small tree ranging from just a few feet tall to more than 30 ft. in height, though its usual height at maturity is 10-15 ft.

Sophora secundiflora is very popular as a native evergreen ornamental tree within its range, valued for its handsome, dark green foliage and lush early spring blooms. It is drought-tolerant, prefers rocky limestone soil, and is native from central Texas west to New Mexico and south to San Luis Potosi in Mexico. Like many woody plants native to rocky soils, it is slow growing. The fragrance of Texas mountain laurel flowers is reminiscent of artificial grape products. Bloom fragrance often compared to artificial grape products like grape Kool-Aid, grape bubble gum, grape soft drinks, etc. A pleasant but almost overpoweringly strong fragrance that can waft a considerable distance from the plant. The brilliant, lacquer red seeds were valued by indigenous people for ornament and ceremonial use; they contain the highly poisonous alkaloid cytisine (or sophorine), a substance related to nicotine and widely cited as a narcotic and hallucinogen.

Now the border plants…

From the Monrovia.com website - Lavandula angustifolia ‘Thumbelina Leigh’

An extremely aromatic and profuse bloomer, this compact selection produces lovely spikes of violet-blue blooms up to three times per year! Shear by one-half after flowering for best repeat show. This versatile, undemanding little shrub is ideal for containers, low borders, and rock gardens. Compact foliage 6-12 inches tall and 12 inches wide. 12-18 inches tall in bloom.

From the Missouri Botanical Garden website - Buxus sinica var. insularis ‘Winter Gem’

‘Winter Gem’ is a Korean boxwood that typically matures to 2-3’ tall and as wide. It is an evergreen shrub that features small, oval-rounded, dark green leaves. Yellowish-green spring flowers are inconspicuous. Synonymous with and sometimes sold in commerce as Buxus microphylla var. japonica ‘Winter Gem’ or Buxus microphylla var. koreana ‘Winter Gem. Best grown in average, medium moisture, well-drained soil in full sun to part shade. Prefers moist, sandy loams with a slightly acidic to slightly alkaline pH in sun-dappled part shade. Tolerant of pruning and shearing. Avoid cultivating around plants because they have shallow roots. Roots appreciate a good mulch (1-2”). Thin plants and remove dead/damaged branches annually to improve air circulation. Carefully remove heavy snow accumulations from plants as quickly as practicable to minimize stem/branch damage.

So to answer your question, plant the Thumbelina Leigh lavender on the outside as it is the shorter plant, then the Winter Gem boxwood (or dwarf boxwoods), then the Texas mountain laurel.